Neurophilosophy

From Wikipedia, the free encyclopedia

Neurophilosophy or philosophy of neuroscience is the interdisciplinary study of neuroscience and philosophy that explores the relevance of neuroscientific studies to the arguments traditionally categorized as philosophy of mind. The philosophy of neuroscience attempts to clarify neuroscientific methods and results using the conceptual rigor and methods of philosophy of science.

Specific issues

Below is a list of specific issues important to philosophy of neuroscience:

• "The indirectness of studies of mind and brain"^[1]
• "Computational or representational analysis of brain processing"^[2]
• "Relations between psychological and neuroscientific inquiries"^[3]
• Modularity of mind^[2]
• What constitutes adequate explanation in neuroscience?^[4]
• "Location of cognitive function"^[5]

The indirectness of studies of mind and brain

Many of the methods and techniques central to neuroscientific discovery rely on assumptions that can limit the interpretation of the data. Philosophers of Neuroscience have discussed such assumptions in the use of functional Magnetic Resonance Imaging,^[6][7] Dissociation in Cognitive Neuropsychology,^[8][9] single unit recording,^[10] and computational neuroscience.^[11] Following are descriptions of many of the current controversies and debates about the methods employed in neuroscience.

fMRI

Many fMRI studies rely heavily on the assumption of "localization of function"^[12](same as functional specialization). Localization of function means that many cognitive functions can be localized to specific brain regions. A good example of functional localization comes from studies of the motor cortex.^[13] There seem to be different groups of cells in the motor cortex responsible for controlling different groups of muscles. Many philosophers of neuroscience criticize fMRI for relying too heavily on this assumption. Michael Anderson points out that subtraction method fMRI misses a lot of brain information that is important to the cognitive processes.^[14] Subtraction fMRI only shows the differences between the task activation and the control activation, but many of the brain areas activated in the control are obviously important for the task as well.

Some philosophers entirely reject any notion of localization of function and thus believe fMRI studies to be profoundly misguided.^[15] These philosophers maintain that brain processing acts holistically, that large sections of the brain are involved in processing most cognitive tasks (see holism in neurology and the modularity section below). One way to understand their objection to the idea of localization of function is the radio repair man thought experiment.^[16] In this thought experiment, a radio repair man opens up a radio and rips out a tube. The radio begins whistling loudly and the radio repair man declares that he must have ripped out the anti-whistling tube. There is not really any anti-whistling tube in the radio and it is obvious that the radio repair man has confounded function with effect. This criticism was originally targeted at the logic used by neuropsychological brain lesion experiments, but the criticism is still applicable to neuroimaging. These considerations are similar to Van Orden's and Paap's criticism of circularity in neuroimaging logic.^[17] According to them, neuroimagers assume that their theory of cognitive component parcellation is correct and that these components divide cleanly into feed-forward modules. These assumptions are necessary to justify their inference of brain localization. The logic is circular if the researcher then use the appearance of brain region activation as proof of the correctness of their cognitive theories.

A different problematic methodological assumption within fMRI research is the use of reverse inference^[18] A reverse inference is when the activation of a brain region is used to infer the presence of a given cognitive process. Poldrack points out that the strength of this inference depends critically on the likelihood that a given task employs a given cognitive process and the likelihood of that pattern of brain activation given that cognitive process. In other words, the strength of reverse inference is based upon the selectivity of the task used as well as the selectivity of the brain region activation. A 2011 article published in the NY times has been heavily criticized for misusing reverse inference.^[19] In the study, participants were shown pictures of their iPhones and the researchers measured activation of the insula. The researches took insula activation as evidence of feelings of love and concluded that people loved their iPhones. Critics were quick to point out that the insula is not a very selective piece of cortex, and therefore not amenable to reverse inference.

The Neuropsychologist Max Coltheart took the problems with reverse inference a step further and challenged neuroimagers to give one instance in which neuroimaging had informed psychological theory^[20] Coltheart takes the burden of proof to be an instance where the brain imaging data is consistent with one theory but inconsistent with another theory. Roskies maintains that Coltheart's ultra cognitive position makes his challenge unwinnable.^[21] Since Coltheart maintains that the implementation of a cognitive state has no bearing on the function of that cognitive state, then it is impossible to find neuroimaging data that will be able to comment on psychological theories in the way Coltheart demands. Neuroimaging data will always be relegated to the lower level of implementation and be unable to selectively determine one or another cognitive theory. In a 2006 article, Richard Henson suggests that forward inference can be used to infer dissociation of function at the psychological level.^[22] He suggests that these kinds of inferences can be made when there is crossing activations between two task types in two brain regions and there is no change in activation in a mutual control region.

One final assumption worth mentioning is the assumption of pure insertion in fMRI.^[23] The assumption of pure insertion is the assumption that a single cognitive process can be inserted into another set of cognitive process without effecting the functioning of the rest. For example, if you wanted to find the reading comprehension area of the brain, you might scan participants while they were presented with a word and while they were presented with a non-word (e.g. "Floob"). If you infer that the resulting difference in brain pattern represents the regions of the brain involved in reading comprehension, you have assumed that these changes are not reflective of changes in task difficulty or differential recruitment between tasks. The term pure insertion was coined by Donders as a criticism of reaction time methods.

Recently, researchers have begun using a new functional imaging technique called resting state functional connectivity MRI.^[24] Subjects' brains are scanned while the subject sits idly in the scanner. By looking at the natural fluctuations in the bold pattern while the subject is at rest, the researchers can see which brain regions co-vary in activation together. They can use the patterns of covariance to construct maps of functionally linked brain areas. It is worth noting that the name "functional connectivity" is somewhat misleading since the data only indicates co-variation. Still, this is a powerful method for studying large networks throughout the brain. There are a couple of important methodological issues that need to be addressed. Firstly, there are many different possible brain mappings that could be used to define the brain regions for the network. The results could vary significantly depending on the brain region chosen. Secondly, what mathematical techniques are best about to characterize these brain regions?

The brain regions of interest are somewhat constrained by the size of the voxels. Rs-fcMRI uses voxels that are few millimeters cubed so the brain regions will have to be defined on a larger scale. Two of the statistical methods that are commonly applied to network analysis can work on the single voxel spatial scale, but graph theory methods are extremely sensitive to the way nodes are defined. Brains regions can be divided according to their cellular architectural, according to their connectivity, or according to physiological measures. Alternatively, you could take a theory neutral approach and randomly divide the cortex into partitions of the size of your choosing. As mentioned earlier, there are several approaches to network analysis once the your brain regions have been defined. Seed based analysis begins with an a priori defined seed region and finds all of the regions that are functionally connected to that region. Wig et al. caution that the resulting network structure will not give any information concerning the inter-connectivity of the identified regions or the relations of those regions to regions other than the seed region. Another approach is to use independent component analysis to create spatio-temporal component maps and the components are sorted by components that carry information of interest and those that are caused by noise. Wigs et al. once again warns that inference of functional brain region communities is difficult under ICA. ICA also has the issue of imposing orthogonality on the data.^[25] Graph theory uses a matrix to characterize covariance between regions which is then transformed into a network map. The problem with graph theory analysis is that network mapping is heavily influenced by a priori brain region and connectivity (nodes and edges), thus the researcher is at risk for cherry picking regions and connections according to their own theories. However, graph theory analysis is extremely valuable since it is the only method that gives pair-wise relationships between nodes. ICA has the added advantage of being a fairly principled method. It seems that using both methods will be important in uncovering the network connectivity of the brain. Mumford et al. hoped to avoid these issues and use a principled approach that could determine pair-wise relationships using a statistical technique adopted from analysis of gene co-expression networks.

Dissociation in cognitive neuropsychology

Cognitive Neuropsychology studies brain damaged patients and uses the patterns of selective impairment in order to make inferences on the underlying cognitive structure. Dissociation between cognitive functions is taken to be evidence that these functions are independent. Theorists have identified several key assumptions that are needed to justify these inferences:^[26] 1) Functional Modularity- the mind is organized into functionally separate cognitive modules. 2). Anatomical Modularity- the brain is organized into functionally separate modules. This assumption is very similar to the assumption of functional localization. These assumptions differ from the assumption of functional modularity, because it is possible to have separable cognitive modules that are implemented by diffuse patterns of brain activation. 3)Universality- The basic organization of functional and anatomical modularity is the same for all normal humans. This assumption is needed if we are to make any claim about functional organization based on dissociation that extrapolates from the instance of a case study to the population. 4) Transparency / Subtractivity- the mind does not undergo substantial reorganization following brain damage. It is possible to remove one functional module without significantly altering the overall structure of the system. This assumption is necessary in order to justify using brain damaged patients in order to make inferences about the cognitive architecture of healthy people.

There are three principal types of evidence in cognitive neuropsychology: association, single dissociation and double dissociation.^[27] Association inferences observe that certain deficits are likely to co-occur. For example, there are many cases who have deficits in both abstract and concrete word comprehension following brain damage. Association studies are considered the weakest form of evidence, because the results could be accounted for by damage to neighboring brain regions and not damage to a single cognitive system.^[28] Single Dissociation inferences observe that one cognitive faculty can be spared while another can be damaged following brain damage. This pattern indicates that a) the two tasks employ different cognitive systems b) the two tasks occupy the same system and the damaged task is downstream from the spared task or c) that the spared task requires fewer cognitive resources than the damaged task. The "gold standard" for cognitive neuropsychology is the double dissociation. Double dissociation occurs when brain damage impairs task A in Patient1 but spares task B and brain damage spares task A in Patient 2 but damages task B. It is assumed that one instance of double dissociation is sufficient proof to infer separate cognitive modules in the performance of the tasks.

Many theorists criticize cognitive neuropsychology for its dependence on double dissociations. In one widely cited study, Joula and Plunkett used a model connectionist system to demonstrate that double dissociation behavioral patterns can occur through random lesions of a single module.^[29] They created a multilayer connectionist system trained to pronounce words. They repeatedly simulated random destruction of nodes and connections in the system and plotted the resulting performance on a scatter plot. The results showed deficits in irregular noun pronunciation with spared regular verb pronunciation in some cases and deficits in regular verb pronunciation with spared irregular noun pronunciation. These results suggest that a single instance of double dissociation is insufficient to justify inference to multiple systems.^[30]

Charter offers a theoretical case in which double dissociation logic can be faulty.^[31] If two tasks, task A and task B, use almost all of the same systems but differ by one mutually exclusive module apiece, then the selective lesioning of those two modules would seem to indicate that A and B use different systems. Charter uses the example of someone who is allergic to peanuts but not shrimp and someone who is allergic to shrimp and not peanuts. He argues that double dissociation logic leads one to infer that peanuts and shrimp are digested by different systems. John Dunn offers another objection to double dissociation.^[32] He claims that it is easy to demonstrate the existence of a true deficit but difficult to show that another function is truly spared. As more data is accumulated, the value of your results will converge on an effect size of zero, but there will always be a positive value greater than zero that has more statistical power than zero. Therefore, it is impossible to be fully confident that a given double dissociation actually exists.

On a different note, Alphonso Caramazza has given a principled reason for rejecting the use of group studies in cognitive neuropsychology.^[33] Studies of brain damaged patients can either take the form of a single case study, in which an individual's behavior is characterized and used as evidence, or group studies, in which a group of patients displaying the same deficit have their behavior characterized and averaged. In order to justify grouping a set of patient data together, the researcher must know that the group is homogenous, that their behavior is equivalent in every theoretically meaningful way. In brain damaged patients, this can only be accomplished a posteriori by analyzing the behavior patterns of all the individuals in the group. Thus according to Caramazza, any group study is either the equivalent of a set of single case studies or is theoretically unjustified. Newcombe and Marshall pointed out that there are some cases (they use Geschwind's syndrome as an example) and that group studies might still serve as a useful heuristic in cognitive neuropsychological studies.^[34]

Single unit recordings

It is commonly understood in neuroscience that information is encoded in the brain by the firing patterns of neurons.^[35] Many of the philosophical questions surrounding the neural code are related to questions about representation and computation that are discussed below. There are other methodological questions including whether neurons represent information through an average firing rate or whether there is information represented by the temporal dynamics. There are similar questions about whether neurons represent information individually or as a population.

Computational neuroscience

Many of the philosophical controversies surrounding computational neuroscience involve the role of simulation and modeling as explanation. Carl Craver has been especially vocal about such interpretations.^[36] Jones and Love wrote an especially critical article targeted at Bayesian behavioral modeling that did not constrain the modeling parameters by psychological or neurological considerations^[37] Eric Winsberg has written about the role of computer modeling and simulation in science generally, but his characterization is applicable to computational neuroscience.^[38]

Computation and representation in the brain

The computational theory of mind has been widespread in neuroscience since the cognitive revolution in the 1960s. This section will begin with a historical overview of computational neuroscience and then discuss various competing theories and controversies within the field.

Historical overview

Computational neuroscience began in the 1930s and 1940s with two groups of researchers. The first group consisted of Alan Turing, Alonzo Church and John von Neumann, who were working to develop computing machines and the mathematical underpinnings of computer science.^[39] This work culminated in the theoretical development of so-called Turing machines and the Church–Turing thesis, which formalized the mathematics underlying computability theory. The second group consisted of Warren McCulloch and Walter Pitts who were working to develop the first artificial neural networks. McCulloch and Pitts were the first to hypothesize that neurons could be used to implement a logical calculus that could explain cognition. They used their toy neurons to develop logic gates that could make computations.^[40] However these developments failed to take hold in the psychological sciences and neuroscience until the mid-1950s and 1960s. Behaviorism had dominated the psychology until the 1950s when new developments in a variety of fields overturned behaviorist theory in favor of a cognitive theory. From the beginning of the cognitive revolution, computational theory played a major role in theoretical developments. Minsky and McCarthy's work in artificial intelligence, Newell and Simon's computer simulations, and Noam Chomsky's importation of information theory into linguistics were all heavily reliant on computational assumptions.^[41] By the early 1960s, Hilary Putnam was arguing in favor of machine functionalism in which the brain instantiated Turing machines. By this point computational theories were firmly fixed in psychology and neuroscience. By the mid-1980s, a group of researchers began using multilayer feed-forward analog neural networks that could be trained to perform a variety of tasks. The work by researchers like Sejnowski, Rosenberg, Rumelhart, and McClelland were labeled as connectionism, and the discipline has continued since then.^[42] The connectionist mindset was embraced by Paul and Patricia Churchland who then developed their "state space semantics" using concepts from connectionist theory. Connectionism was also condemned by researchers such as Fodor, Pylyshyn, and Pinker. The tension between the connectionists and the classicists is still being debated today.

Representation

One of the reasons that computational theories are appealing is that computers have the ability to manipulate representations to give meaningful output. Digital computers use strings of 1s and 0s in order to represent the content such as this Wikipedia page. Most cognitive scientists posit that our brains use some form of representational code that is carried in the firing patterns of neurons. Computational accounts seem to offer an easy way of explaining how our brains carry and manipulate the perceptions, thoughts, feelings, and actions that make up our everyday experience.^[43] While most theorists maintain that representation is an important part of cognition, the exact nature of that representation is highly debated. The two main arguments come from advocates of symbolic representations and advocates of associationist representations.

Symbolic representational accounts have been famously championed by Fodor and Pinker. Symbolic representation means that the objects are represented by symbols and are processed through rule governed manipulations that are sensation to the constitutive structure. The fact that symbolic representation is sensitive to the structure of the representations is a major part of its appeal. Fodor proposed the Language of Thought Hypothesis in which mental representations manipulated in the same way that language is syntactically manipulated in order to produce thought. According to Fodor, the language of thought hypothesis explains the systematicity and productivity seen in both language and thought.^[44] Blechner^[45] proposed that dreams reveal a figurative-affective representation that may represent the language of thought or, more properly, the substrate of thought. Blechner’s model accounts for how evolutionarily earlier forms of humans and non-human mammals could think, dream, and solve problems without language. Waking thought became a later development in which underlying figurative-affective thinking is transformed into communicable linguistic forms. It would also account for the way problem-solving can occur in dreams.^[46]

Associativist representations are most often described with connectionist systems. In connectionist systems, representations are distributed across all the nodes and connection weights of the system and thus are said to be sub symbolic.^[47] It is worth noting that a connectionist system is capable of implementing a symbolic system. There are several important aspects of neural nets that suggest that distributed parallel processing provides a better basis for cognitive functions than symbolic processing. Firstly, the inspiration for these systems came from the brain itself indicating biological relevance. Secondly, these systems are capable of storing content addressable memory, which is far more efficient than memory searches in symbolic systems. Thirdly, neural nets are resilient to damage while even minor damage can disable a symbolic system. Lastly, soft constraints and generalization when processing novel stimuli allow nets to behave more flexibly than symbolic systems.

The Churchlands described representation in a connectionist system in terms of state space. The content of the system is represented by an n-dimensional vector where the n= the number of nodes in the system and the direction of the vector is determined by the activation pattern of the nodes. Fodor rejected this method of representation on the grounds that two different connectionist systems could not have the same content.^[48] Further mathematical analysis of connectionist system relieved that connectionist systems that could contain similar content could be mapped graphically to reveal clusters of nodes that were important to representing the content.^[49] Unfortunately for the Churchlands, state space vector comparison was not amenable to this type of analysis. Recently, Nicholas Shea has offered his own account for content within connectionist systems that employs the concepts developed through cluster analysis.

Views on computation

Computational neuroscience is committed to the position that the brain is some sort of computer, but what does it mean to be a computer? The definition of a computation must be narrow enough so that we limit the number of objects that can be called computers. For example, it might seem problematic to have a definition wide enough to allow stomachs and weather systems to be involved in computations. However, it is also necessary to have a definition broad enough to allow all of the wide varieties of computational systems to compute. For example, if the definition of computation is limited to syntactic manipulation of symbolic representations, then most connectionist systems would not be able to compute.^[50] Rick Grush distinguishes between computation as a tool for simulation and computation as a theoretical stance in cognitive neuroscience.^[51] For the former, anything that can be computationally modeled counts as computing. In the latter case, the brain is a computing function that is distinct from systems like fluid dynamic systems and the planetary orbits in this regard. The challenge for any computational definition is to keep the two senses distinct.

Alternatively, some theorists choose to accept a narrow or wide definition for theoretical reasons. Pancomputationalism is the position that everything can be said to compute. This view has been criticized by Piccinini on the grounds that such a definition makes computation trivial to the point where it is robbed of its explanatory value.^[52]

The simplest definition of computations is that a system can be said to be computing when a computational description can be mapped onto the physical description. This is an extremely broad definition of computation and it ends up endorsing a form of pancomputationalism. Putnam and Searle, who are often credited with this view, maintain that computation is observer-related. In other words, if you want to view a system as computing then you can say that it is computing. Piccinini points out that, in this view, not only is everything computing, but also everything is computing in an indefinite number of ways.^[53] Since it is possible to apply an indefinite number of computational descriptions to a given system, the system ends up computing an indefinite number of tasks.

The most common view of computation is the semantic account of computation. Semantic approaches use a similar notion of computation as the mapping approaches with the added constraint that the system must manipulate representations with semantic content. Note from the earlier discussion of representation that both the Churchlands' connectionist systems and Fodor's symbolic systems use this notion of computation. In fact, Fodor is famously credited as saying "No computation without representation".^[54] Computational states can be individuated by an externalized appeal to content in a broad sense (i.e. the object in the external world) or by internalist appeal to the narrow sense content (content defined by the properties of the system).^[55] In order to fix the content of the representation, it is often necessary to appeal to the information contained within the system. Grush provides a criticism of the semantic account.^[51] He points out that appeal to the informational content of a system to demonstrate representation by the system. He uses his coffee cup as an example of a system that contains information, such as the heat conductance of the coffee cup and the time since the coffee was poured, but is too mundane to compute in any robust sense. Semantic computationalists try to escape this criticism by appealing to the evolutionary history of system. This is called the biosemantic account. Grush uses the example of his feet, saying that by this account his feet would not be computing the amount of food he had eaten because their structure had not been evolutionarily selected for that purpose. Grush replies to the appeal to biosemantics with a thought experiment. Imagine that lightning strikes a swamp somewhere and creates an exact copy of you. According to the biosemantic account, this swamp-you would be incapable of computation because there is no evolutionary history with which to justify assigning representational content. The idea that for two physically identical structures one can be said to be computing while the other is not should be disturbing to any physicalist.

There are also syntactic or structural accounts for computation. These accounts do not need to rely on representation. However, it is possible to use both structure and representation as constrains on computational mapping. Shagrir identifies several philosophers of neuroscience who espouse structural accounts. According to him, Fodor and Pylyshyn require some sort of syntactic constraint on their theory of computation. This is consistent with their rejection of connectionist systems on the grounds of systematicity. He also identifies Piccinini as a structuralist quoting his 2008 paper: "the generation of output strings of digits from input strings of digits in accordance with a general rule that depends on the properties of the strings and (possibly) on the internal state of the system".^[56] Though Piccinini undoubtedly espouses structuralist views in that paper, he claims that mechanistic accounts of computation avoid reference to either syntax or representation.^[55] It is possible that Piccinini thinks that there are differences between syntactic and structural accounts of computation that Shagrir does not respect.

In his view of mechanistic computation, Piccinini asserts that functional mechanisms process vehicles in a manner sensitive to the differences between different portions of the vehicle, and thus can be said to generically compute. He claims that these vehicles are medium-independent, meaning that the mapping function will be the same regardless of the physical implementation. Computing systems can be differentiated based upon the vehicle structure and the mechanistic perspective can account for errors in computation.

Dynamical systems theory presents itself as an alternative to computational explanations of cognition. These theories are staunchly anti-computational and anti-representational. Dynamical systems are defined as systems that change over time in accordance with a mathematical equation. Dynamical systems theory claims that human cognition is a dynamical model in the same sense computationalists claim that the human mind is a computer.^[57] A common objection leveled at dynamical systems theory is that dynamical systems are computable and therefore a subset of computationalism. Van Gelder is quick to point out that there is a big difference between being a computer and being computable. Making the definition of computing wide enough to incorporate dynamical models would effectively embrace pancomputationalism.

Mind–body dualism

From Wikipedia, the free encyclopedia

René Descartes's illustration of dualism. Inputs are passed on by the sensory organs to the epiphysis in the brain and from there to the immaterial spirit.

Mind–body dualism, or mind–body duality, is a view in the philosophy of mind that mental phenomena are, in some respects, non-physical, or that the mind and body are distinct and separable. Thus, it encompasses a set of views about the relationship between mind and matter, and between subject and object, and is contrasted with other positions, such as physicalism and enactivism, in the mind–body problem.

Aristotle shared Plato's view of multiple souls and further elaborated a hierarchical arrangement, corresponding to the distinctive functions of plants, animals, and people: a nutritive soul of growth and metabolism that all three share; a perceptive soul of pain, pleasure, and desire that only people and other animals share; and the faculty of reason that is unique to people only. In this view, a soul is the hylomorphic form of a viable organism, wherein each level of the hierarchy formally supervenes upon the substance of the preceding level. Thus, for Aristotle, all three souls perish when the living organism dies.^[3][4] For Plato however, the soul was not dependent on the physical body; he believed in metempsychosis, the migration of the soul to a new physical body.^[5]

Dualism is closely associated with the thought of René Descartes (1641), which holds that the mind is a nonphysical—and therefore, non-spatial—substance. Descartes clearly identified the mind with consciousness and self-awareness and distinguished this from the brain as the seat of intelligence.^[6] Hence, he was the first to formulate the mind–body problem in the form in which it exists today.^[7] Dualism is contrasted with various kinds of monism. Substance dualism is contrasted with all forms of materialism, but property dualism may be considered a form of emergent materialism or non-reductive physicalism in some sense.

Types

Ontological dualism makes dual commitments about the nature of existence as it relates to mind and matter, and can be divided into three different types:

1. Substance dualism asserts that mind and matter are fundamentally distinct kinds of foundations.^[1]
2. Property dualism suggests that the ontological distinction lies in the differences between properties of mind and matter (as in emergentism).^[1]
3. Predicate dualism claims the irreducibility of mental predicates to physical predicates.^[1]

Substance or Cartesian dualism

Substance dualism is a type of dualism most famously defended by René Descartes, which states that there are two kinds of foundation: mental and body.^[6] This philosophy states that the mental can exist outside of the body, and the body cannot think. Substance dualism is important historically for having given rise to much thought regarding the famous mind–body problem. Substance dualism is a philosophical position compatible with most theologies which claim that immortal souls occupy an independent realm of existence distinct from that of the physical world.^[1]

Property dualism

Property dualism asserts that an ontological distinction lies in the differences between properties of mind and matter, and that consciousness is ontologically irreducible to neurobiology and physics. It asserts that when matter is organized in the appropriate way (i.e., in the way that living human bodies are organized), mental properties emerge. Hence, it is a sub-branch of emergent materialism. What views properly fall under the property dualism rubric is itself a matter of dispute. There are different versions of property dualism, some of which claim independent categorisation.^[8]

Non-reductive physicalism is a form of property dualism in which it is asserted that all mental states are causally reducible to physical states. One argument for this has been made in the form of anomalous monism expressed by Donald Davidson, where it is argued that mental events are identical to physical events, and there can be strict law-governed causal relationships. Another argument for this has been expressed by John Searle, who is the advocate of a distinctive form of physicalism he calls biological naturalism. His view is that although mental states are ontologically irreducible to physical states, they are causally reducible (see causality). He has acknowledged that "to many people" his views and those of property dualists look a lot alike. But he thinks the comparison is misleading.^[8]

Epiphenomenalism

Epiphenomenalism is a form of Property Dualism, in which it is asserted that one or more mental states do not have any influence on physical states (both ontologically and causally irreducible). It asserts that while material causes give rise to sensations, volitions, ideas, etc., such mental phenomena themselves cause nothing further: they are causal dead-ends. This can be contrasted to interactionism, on the other hand, in which mental causes can produce material effects, and vice versa.^[9]

Predicate dualism

Predicate dualism is a view espoused by nonreductive physicalists such as Donald Davidson and Jerry Fodor, who maintain that while there is only one ontological category of substances and properties of substances (usually physical), the predicates that we use to describe mental events cannot be redescribed in terms of (or reduced to) physical predicates of natural languages.^[10][11] If we characterize predicate monism as the view subscribed to by eliminative materialists, who maintain that such intentional predicates as believe, desire, think, feel, etc., will eventually be eliminated from both the language of science and from ordinary language because the entities to which they refer do not exist, then predicate dualism is most easily defined as the negation of this position. Predicate dualists believe that so-called "folk psychology", with all of its propositional attitude ascriptions, is an ineliminable part of the enterprise of describing, explaining and understanding human mental states and behavior.

Davidson, for example, subscribes to Anomalous Monism, according to which there can be no strict psycho-physical laws which connect mental and physical events under their descriptions as mental and physical events. However, all mental events also have physical descriptions. It is in terms of the latter that such events can be connected in law-like relations with other physical events. Mental predicates are irreducibly different in character (rational, holistic and necessary) from physical predicates (contingent, atomic and causal).^[10]

Dualist views of mental causation

Four varieties of dualist causal interaction. The arrows indicate the direction of causations. Mental and physical states are shown in red and blue, respectively.

This part is about causation between properties and states of the thing under study, not its substances or predicates. Here a state is the set of all properties of what's being studied. Thus each state describes only one point in time.

Interactionism

Interactionism is the view that mental states, such as beliefs and desires, causally interact with physical states. This is a position which is very appealing to common-sense intuitions, notwithstanding the fact that it is very difficult to establish its validity or correctness by way of logical argumentation or empirical proof. It seems to appeal to common-sense because we are surrounded by such everyday occurrences as a child's touching a hot stove (physical event) which causes him to feel pain (mental event) and then yell and scream (physical event) which causes his parents to experience a sensation of fear and protectiveness (mental event) and so on.^[6]

Non-reductive physicalism

Non-reductive physicalism is the idea that while mental states are physical they are not reducible to physical properties, in that an ontological distinction lies in the differences between the properties of mind and matter. According to non-reductive physicalism all mental states are causally reducible to physical states where mental properties map to physical properties and vice versa. A prominent form of non-reductive physicalism called anomalous monism was first proposed by Donald Davidson in his 1970 paper Mental events, where it is claimed that mental events are identical with physical events, and that the mental is anomalous, i.e. under their mental descriptions these mental events are not regulated by strict physical laws.

Epiphenomenalism

Epiphenomenalism states that all mental events are caused by a physical event and have no physical consequences, and that one or more mental states do not have any influence on physical states. So, the mental event of deciding to pick up a rock ("M1") is caused by the firing of specific neurons in the brain ("P1"). When the arm and hand move to pick up the rock ("P2") this is not caused by the preceding mental event M1, nor by M1 and P1 together, but only by P1. The physical causes are in principle reducible to fundamental physics, and therefore mental causes are eliminated using this reductionist explanation. If P1 causes both M1 and P2, there is no overdetermination in the explanation for P2.^[6]

The idea that even if the animal were conscious nothing would be added to the production of behavior, even in animals of the human type, was first voiced by La Mettrie (1745), and then by Cabanis (1802), and was further explicated by Hodgson (1870) and Huxley (1874).^[12] Jackson gave a subjective argument for epiphenomenalism, but later refuted it and embraced physicalism.^[13]

Parallelism

Psycho-physical parallelism is a very unusual view about the interaction between mental and physical events which was most prominently, and perhaps only truly, advocated by Gottfried Wilhelm von Leibniz. Like Malebranche and others before him, Leibniz recognized the weaknesses of Descartes' account of causal interaction taking place in a physical location in the brain. Malebranche decided that such a material basis of interaction between material and immaterial was impossible and therefore formulated his doctrine of occasionalism, stating that the interactions were really caused by the intervention of God on each individual occasion. Leibniz's idea is that God has created a pre-established harmony such that it only seems as if physical and mental events cause, and are caused by, one another. In reality, mental causes only have mental effects and physical causes only have physical effects. Hence the term parallelism is used to describe this view.^[9]

Occasionalism

Occasionalism is a philosophical doctrine about causation which says that created substances cannot be efficient causes of events. Instead, all events are taken to be caused directly by God himself. The theory states that the illusion of efficient causation between mundane events arises out of a constant conjunction that God had instituted, such that every instance where the cause is present will constitute an "occasion" for the effect to occur as an expression of the aforementioned power. This "occasioning" relation, however, falls short of efficient causation. In this view, it is not the case that the first event causes God to cause the second event: rather, God first caused one and then caused the other, but chose to regulate such behaviour in accordance with general laws of nature. Some of its most prominent historical exponents have been Louis de la Forge, Arnold Geulincx, and Nicholas Malebranche.^[14]

Kantianism

According to Immanuel Kant's philosophy, there is a distinction between actions done by desire and the ones performed by liberty (categorical imperative). Thus, not all physical actions are caused by either matter or freedom. Some actions are purely animal in nature, while others are the result of mental action on matter.

Historical overview

Plato and Aristotle

In the dialogue Phaedo, Plato formulated his famous Theory of Forms as distinct and immaterial substances of which the objects and other phenomena that we perceive in the world are nothing more than mere shadows.^[5]

Plato makes it clear, in the Phaedo, that the Forms are the universalia ante res, i.e. they are ideal universals, by which we are able to understand the world. In his allegory of the cave Plato likens the achievement of philosophical understanding to emerging into the sun from a dark cave, where only vague shadows of what lies beyond that prison are cast dimly upon the wall. Plato's forms are non-physical and non-mental. They exist nowhere in time or space, but neither do they exist in the mind, nor in the pleroma of matter; rather, matter is said to "participate" in form (μεθεξις methexis). It remained unclear however, even to Aristotle, exactly what Plato intended by that.

Aristotle argued at length against many aspects of Plato's forms, creating his own doctrine of hylomorphism wherein form and matter coexist. Ultimately however, Aristotle's aim was to perfect a theory of forms, rather than to reject it. Although Aristotle strongly rejected the independent existence Plato attributed to forms, his metaphysics do agree with Plato's a priori considerations quite often. For example, Aristotle argues that changeless, eternal substantial form is necessarily immaterial. Because matter provides a stable substratum for a change in form, matter always has the potential to change. Thus, if given an eternity in which to do so, it will, necessarily, exercise that potential.

Part of Aristotle's psychology, the study of the soul, is his account of the ability of humans to reason and the ability of animals to perceive. In both cases, perfect copies of forms are acquired, either by direct impression of environmental forms, in the case of perception, or else by virtue of contemplation, understanding and recollection. He believed the mind can literally assume any form being contemplated or experienced, and it was unique in its ability to become a blank slate, having no essential form. As thoughts of earth are not heavy, any more than thoughts of fire are causally efficient, they provide an immaterial complement for the formless mind.^[3]

From Neoplatonism to scholasticism

In the philosophical school of Neoplatonism, most active in Late Antiquity, claimed that the physical and the spiritual are both emanations of the One. Neoplatonism exerted a considerable influence on Christianity, as did the philosophy of Aristotle via scholasticism.^[15]

In the scholastic tradition of Saint Thomas Aquinas, a number of whose doctrines have been incorporated into Roman Catholic dogma, the soul is the substantial form of a human being.^[16] Aquinas held the Quaestiones disputate de anima, or "Disputed questions on the soul", at the Roman studium provinciale of the Dominican Order at Santa Sabina, the forerunner of the Pontifical University of Saint Thomas Aquinas, Angelicum during the academic year 1265-66.^[17] By 1268 Aquinas had written at least the first book of the Sententia Libri De anima, Aquinas' commentary on Aristotle's De anima, the translation of which from the Greek was completed by Aquinas' Dominican associate at Viterbo William of Moerbeke in 1267.^[18] Like Aristotle, Aquinas held that the human being was a unified composite substance of two substantial principles: form and matter. The soul is the substantial form and so the first actuality of a material organic body with the potentiality for life.^[19] While Aquinas defended the unity of human nature as a composite substance constituted by these two inextricable principles of form and matter, he also argued for the incorruptibility of the intellectual soul,^[20] in contrast to the corruptibility of the vegetative and sensitive animation of plants and animals.^[21] His argument for the subsistence and incorruptibility of the intellectual soul takes its point of departure from the metaphysical principle that operation follows upon being (agiture sequitur esse), i.e., the activity of a thing reveals the mode of being and existence it depends upon. Since the intellectual soul exercises its own per se intellectual operations without employing material faculties, i.e. intellectual operations are immaterial, the intellect itself and the intellectual soul, must likewise be immaterial and so incorruptible. Even though the intellectual soul of man is able to subsist upon the death of the human being, Aquinas does not hold that the human person is able to remain integrated at death. The separated intellectual soul is neither a man nor a human person. The intellectual soul by itself is not a human person (i.e., an individual supposit of a rational nature).^[22] Hence, Aquinas held that "soul of St. Peter pray for us" would be more appropriate than "St. Peter pray for us", because all things connected with his person, including memories, ended with his corporeal life.^[23]

The Catholic doctrine of the resurrection of the body does nor subscribe that, sees body and soul as forming a whole and states that at the second coming, the souls of the departed will be reunited with their bodies as a whole person (substance) and witness to the apocalypse. The thorough consistency between dogma and contemporary science was maintained here^[24] in part from a serious attendance to the principle that there can be only one truth. Consistency with science, logic, philosophy, and faith remained a high priority for centuries, and a university doctorate in theology generally included the entire science curriculum as a prerequisite. This doctrine is not universally accepted by Christians today. Many believe that one's immortal soul goes directly to Heaven upon death of the body.^[25]

Descartes and his disciples

In his Meditations on First Philosophy, Descartes embarked upon a quest in which he called all his previous beliefs into doubt, in order to find out of what he could be certain.^[7] In so doing, he discovered that he could doubt whether he had a body (it could be that he was dreaming of it or that it was an illusion created by an evil demon), but he could not doubt whether he had a mind. This gave Descartes his first inkling that the mind and body were different things. The mind, according to Descartes, was a "thinking thing" (Latin: res cogitans), and an immaterial substance. This "thing" was the essence of himself, that which doubts, believes, hopes, and thinks. The body, "the thing that exists" (Latin: res extensa), regulates normal bodily functions (such as heart and liver). According to Descartes, animals only had a body and not a soul (which distinguishes humans from animals). The distinction between mind and body is argued in Meditation VI as follows: I have a clear and distinct idea of myself as a thinking, non-extended thing, and a clear and distinct idea of body as an extended and non-thinking thing. Whatever I can conceive clearly and distinctly, God can so create.

The central claim of what is often called Cartesian dualism, in honor of Descartes, is that the immaterial mind and the material body, while being ontologically distinct substances, causally interact. This is an idea that continues to feature prominently in many non-European philosophies. Mental events cause physical events, and vice versa. But this leads to a substantial problem for Cartesian dualism: How can an immaterial mind cause anything in a material body, and vice versa? This has often been called the "problem of interactionism."

Descartes himself struggled to come up with a feasible answer to this problem. In his letter to Elisabeth of Bohemia, Princess Palatine, he suggested that spirits interacted with the body through the pineal gland, a small gland in the centre of the brain, between the two hemispheres.^[7] The term "Cartesian dualism" is also often associated with this more specific notion of causal interaction through the pineal gland. However, this explanation was not satisfactory: how can an immaterial mind interact with the physical pineal gland? Because Descartes' was such a difficult theory to defend, some of his disciples, such as Arnold Geulincx and Nicholas Malebranche, proposed a different explanation: That all mind–body interactions required the direct intervention of God. According to these philosophers, the appropriate states of mind and body were only the occasions for such intervention, not real causes. These occasionalists maintained the strong thesis that all causation was directly dependent on God, instead of holding that all causation was natural except for that between mind and body.^[14]

Recent formulations

During the 19th and 20th centuries, materialistic monism became the norm.^[26] Still, in addition to already discussed theories of dualism (particularly the Christian and Cartesian models) there are new theories in the defense of dualism. Naturalistic dualism comes from Australian philosopher, David Chalmers (born 1966) who argues there is an explanatory gap between objective and subjective experience that cannot be bridged by reductionism because consciousness is, at least, logically autonomous of the physical properties upon which it supervenes. According to Chalmers, a naturalistic account of property dualism requires a new fundamental category of properties described by new laws of supervenience; the challenge being analogous to that of understanding electricity based on the mechanistic and Newtonian models of materialism prior to Maxwell's equations.

A similar defense comes from Australian philosopher Frank Jackson (born 1943) who revived the theory of epiphenomenalism which argues that mental states do not play a role in physical states. Jackson argues that there are two kinds of dualism. The first is substance dualism that assumes there is second, non-corporeal form of reality. In this form, body and soul are two different substances. The second form is property dualism that says that body and soul are different properties of the same body. He claims that functions of the mind/soul are internal, very private experiences that are not accessible to observation by others, and therefore not accessible by science (at least not yet). We can know everything, for example, about a bat's facility for echolocation, but we will never know how the bat experiences that phenomenon.

Arguments for dualism

Another one of Descartes' illustrations. The fire displaces the skin, which pulls a tiny thread, which opens a pore in the ventricle (F) allowing the "animal spirit" to flow through a hollow tube, which inflates the muscle of the leg, causing the foot to withdraw.

The subjective argument

An important fact is that minds perceive intramental states differently from sensory phenomena,^[27] and this cognitive difference results in mental and physical phenomena having seemingly disparate properties. The subjective argument holds that these properties are irreconcilable under a physical mind.

Mental events have a certain subjective quality to them, whereas physical seem not to. So, for example, one may ask what a burned finger feels like, or what the blueness of the sky looks like, or what nice music sounds like.^[28] Philosophers of mind call the subjective aspects of mental events qualia. There is something that it's like to feel pain, to see a familiar shade of blue, and so on. There are qualia involved in these mental events. And the claim is that qualia cannot be reduced to anything physical.^[1]

Thomas Nagel first characterized the problem of qualia for physicalistic monism in his article, "What Is It Like to Be a Bat?". Nagel argued that even if we knew everything there was to know from a third-person, scientific perspective about a bat's sonar system, we still wouldn't know what it is like to be a bat. However, others argue that qualia are consequent of the same neurological processes that engender the bat's mind, and will be fully understood as the science develops.^[29]

Frank Jackson formulated his well-known knowledge argument based upon similar considerations. In this thought experiment, known as Mary's room, he asks us to consider a neuroscientist, Mary, who was born, and has lived all of her life, in a black and white room with a black and white television and computer monitor where she collects all the scientific data she possibly can on the nature of colours. Jackson asserts that as soon as Mary leaves the room, she will come to have new knowledge which she did not possess before: the knowledge of the experience of colours (i.e., what they are like). Although Mary knows everything there is to know about colours from an objective, third-person perspective, she has never known, according to Jackson, what it was like to see red, orange, or green. If Mary really learns something new, it must be knowledge of something non-physical, since she already knew everything about the physical aspects of colour.^[30]

However, Jackson later rejected his argument and embraced physicalism.^[31] He notes that Mary obtains knowledge not of color, but of a new intramental state, seeing color.^[13] Also, he notes that Mary might say "wow," and as a mental state affecting the physical, this clashed with his former view of epiphenomenalism. David Lewis' response to this argument, now known as the ability argument, is that what Mary really came to know was simply the ability to recognize and identify color sensations to which she had previously not been exposed.^[32] Daniel Dennett and others also provide arguments against this notion.

The zombie argument

The zombie argument is based on a thought experiment proposed by David Chalmers. The basic idea is that one can imagine, and, therefore, conceive the existence of, an apparently functioning human being/body without any conscious states being associated with it.

Chalmers' argument is that it seems plausible that such a being could exist because all that is needed is that all and only the things that the physical sciences describe and observe about a human being must be true of the zombie. None of the concepts involved in these sciences make reference to consciousness or other mental phenomena, and any physical entity can be described scientifically via physics whether it is conscious or not. The mere logical possibility of a p-zombie demonstrates that consciousness is a natural phenomenon beyond the current unsatisfactory explanations. Chalmers states that one probably could not build a living p-zombie because living things seem to require a level of consciousness. However (unconscious?) robots built to simulate humans may become the first real p-zombies. Hence Chalmers half-joking calls for the need to build a "consciousness meter" to ascertain if any given entity, human or robot, is conscious or not.^[33][34]

Others such as Dennett have argued that the notion of a philosophical zombie is an incoherent,^[35] or unlikely,^[36] concept. In particular, nothing proves that an entity (e.g., a computer or robot) which would perfectly mimic human beings, and especially perfectly mimic expressions of feelings (like joy, fear, anger, ...), would not indeed experience them, thus having similar states of consciousness to what a real human would have. It is argued that under physicalism, one must either believe that anyone including oneself might be a zombie, or that no one can be a zombie—following from the assertion that one's own conviction about being (or not being) a zombie is a product of the physical world and is therefore no different from anyone else's.

Special sciences argument

Robinson argues that, if predicate dualism is correct, then there are "special sciences" that are irreducible to physics. These allegedly irreducible subjects, which contain irreducible predicates, differ from hard sciences in that they are interest-relative. Here, interest-relative fields depend on the existence of minds that can have interested perspectives.^[9] Psychology is one such science; it completely depends on and presupposes the existence of the mind.

Physics is the general analysis of nature, conducted in order to understand how the universe behaves. On the other hand, the study of meteorological weather patterns or human behavior is only of interest to humans themselves. The point is that having a perspective on the world is a psychological state. Therefore, the special sciences presuppose the existence of minds which can have these states. If one is to avoid ontological dualism, then the mind that has a perspective must be part of the physical reality to which it applies its perspective. If this is the case, then in order to perceive the physical world as psychological, the mind must have a perspective on the physical. This, in turn, presupposes the existence of mind.^[9]

However, cognitive science^[37] and psychology^[38] do not require the mind to be irreducible, and operate on the assumption that it has physical basis. In fact, it is common in science to presuppose a complex system;^[39] while fields such as chemistry,^[40] biology,^[41] or geology^[42] could be verbosely expressed in terms of quantum field theory, it is convenient to use levels of abstraction like molecules, cells, or the mantle. It is often difficult to decompose these levels without heavy analysis^[43] and computation.^[44] Sober has also advanced philosophical arguments against the notion of irreducibility.^[45]

Argument from personal identity

This argument concerns the differences between the applicability of counterfactual conditionals to physical objects, on the one hand, and to conscious, personal agents on the other.^[46] In the case of any material object, e.g. a printer, we can formulate a series of counterfactuals in the following manner:

1. This printer could have been made of straw.
2. This printer could have been made of some other kind of plastics and vacuum-tube transistors.
3. This printer could have been made of 95% of what it is actually made of and 5% vacuum-tube transistors, etc..

Somewhere along the way from the printer's being made up exactly of the parts and materials which actually constitute it to the printer's being made up of some different matter at, say, 20%, the question of whether this printer is the same printer becomes a matter of arbitrary convention.

Imagine the case of a person, Frederick, who has a counterpart born from the same egg and a slightly genetically modified sperm. Imagine a series of counterfactual cases corresponding to the examples applied to the printer. Somewhere along the way, one is no longer sure about the identity of Frederick. In this latter case, it has been claimed, overlap of constitution cannot be applied to the identity of mind. As Madell puts it:

"But while my present body can thus have its partial counterpart in some possible world, my present consciousness cannot. Any present state of consciousness that I can imagine either is or is not mine. There is no question of degree here."^[46]

If the counterpart of Frederick, Frederickus, is 70% constituted of the same physical substance as Frederick, does this mean that it is also 70% mentally identical with Frederick? Does it make sense to say that something is mentally 70% Frederick?^[47] A possible solution to this dilemma is that of open individualism.

Argument from reason

Philosophers and scientists such as Victor Reppert, William Hasker, and Alvin Plantinga have developed an argument for dualism dubbed the "argument from reason". They credit C.S. Lewis with first bringing the argument to light in his book Miracles; Lewis called the argument "The Cardinal Difficulty of Naturalism", which was the title of chapter three of Miracles.^[48]
The argument postulates that if, as naturalism entails, all of our thoughts are the effect of a physical cause, then we have no reason for assuming that they are also the consequent of a reasonable ground. However, knowledge is apprehended by reasoning from ground to consequent. Therefore, if naturalism were true, there would be no way of knowing it (or anything else), except by a fluke.^[48]
Through this logic, the statement "I have reason to believe naturalism is valid" is inconsistent in the same manner as "I never tell the truth."^[49] That is, to conclude its truth would eliminate the grounds from which to reach it. To summarize the argument in the book, Lewis quotes J. B. S. Haldane, who appeals to a similar line of reasoning:^[50]

If my mental processes are determined wholly by the motions of atoms in my brain, I have no reason to suppose that my beliefs are true ... and hence I have no reason for supposing my brain to be composed of atoms.

— J. B. S. Haldane, Possible Worlds, page 209

In his essay "Is Theology Poetry?", Lewis himself summarises the argument in a similar fashion when he writes:

If minds are wholly dependent on brains, and brains on biochemistry, and biochemistry (in the long run) on the meaningless flux of the atoms, I cannot understand how the thought of those minds should have any more significance than the sound of the wind in the trees.

— C. S. Lewis, The Weight of Glory and Other Addresses, page 139

But Lewis later agreed with Elizabeth Anscombe's response to his Miracles argument.^[51] She showed that an argument could be valid and ground-consequent even if its propositions were generated via physical cause and effect by non-rational factors.^[52] Similar to Anscombe, Richard Carrier and John Beversluis have written extensive objections to the argument from reason on the untenability of its first postulate.^[53]

Arguments against dualism

Causal interaction

Cartesian dualism compared to three forms of monism.

One argument against Dualism is with regard to causal interaction. If consciousness (the mind) can exist independently of physical reality (the brain), one must explain how physical memories are created concerning consciousness. Dualism must therefore explain how consciousness affects physical reality. One of the main objections to dualistic interactionism is lack of explanation of how the material and immaterial are able to interact. Varieties of dualism according to which an immaterial mind causally affects the material body and vice versa have come under strenuous attack from different quarters, especially in the 20th century. Critics of dualism have often asked how something totally immaterial can affect something totally material—this is the basic problem of causal interaction.

First, it is not clear where the interaction would take place. For example, burning one's finger causes pain. Apparently there is some chain of events, leading from the burning of skin, to the stimulation of nerve endings, to something happening in the peripheral nerves of one's body that lead to one's brain, to something happening in a particular part of one's brain, and finally resulting in the sensation of pain. But pain is not supposed to be spatially locatable. It might be responded that the pain "takes place in the brain." But evidently, the pain is in the finger. This may not be a devastating criticism.

However, there is a second problem about the interaction. Namely, the question of how the interaction takes place, where in dualism "the mind" is assumed to be non-physical and by definition outside of the realm of science. The mechanism which explains the connection between the mental and the physical would therefore be a philosophical proposition as compared to a scientific theory. For example, compare such a mechanism to a physical mechanism that is well understood. Take a very simple causal relation, such as when a cue ball strikes an eight ball and causes it to go into the pocket. What happens in this case is that the cue ball has a certain amount of momentum as its mass moves across the pool table with a certain velocity, and then that momentum is transferred to the eight ball, which then heads toward the pocket. Compare this to the situation in the brain, where one wants to say that a decision causes some neurons to fire and thus causes a body to move across the room. The intention to "cross the room now" is a mental event and, as such, it does not have physical properties such as force. If it has no force, then it would seem that it could not possibly cause any neuron to fire. However, with Dualism, an explanation is required of how something without any physical properties has physical effects.^[54]

Replies to the argument

Alfred North Whitehead and, later, David Ray Griffin framed a new ontology (process philosophy) seeking precisely to avoid the pitfalls of ontological dualism.^[55]

The explanation given by Arnold Geulincx and Nicholas Malebranche is that of occasionalism, where all mind–body interactions require the direct intervention of God.

At the time C. S. Lewis wrote Miracles,^[56] quantum mechanics (and physical indeterminism) was only in the initial stages of acceptance, but still Lewis stated the logical possibility that, if the physical world was proved to be indeterministic, this would provide an entry (interaction) point into the traditionally viewed closed system, where a scientifically described physically probable/improbable event could be philosophically described as an action of a non-physical entity on physical reality. He states, however, that none of the arguments in his book will rely on this. It should also be noted that, while some interpretations of quantum mechanics consider wave function collapse to be indeterminate,^[57] in others this event is defined and deterministic.^[57]

Argument from physics

The argument from physics is closely related to the argument from causal interaction. Many physicists and consciousness researchers have argued that any action of a nonphysical mind on the brain would entail the violation of physical laws, such as the conservation of energy.^{[58][59][60][61]}

By assuming a deterministic physical universe, the objection can be formulated more precisely. When a person decides to walk across a room, it is generally understood that the decision to do so, a mental event, immediately causes a group of neurons in that person's brain to fire, a physical event, which ultimately results in his walking across the room. The problem is that if there is something totally nonphysical causing a bunch of neurons to fire, then there is no physical event which causes the firing. This means that some physical energy is required to be generated against the physical laws of the deterministic universe—this is by definition a miracle and there can be no scientific explanation of (repeatable experiment performed regarding) where the physical energy for the firing came from.^[62] Such interactions would violate the fundamental laws of physics. In particular, if some external source of energy is responsible for the interactions, then this would violate the law of the conservation of energy.^[63] Dualistic interactionism has therefore been criticized for violating a general heuristic principle of science: the causal closure of the physical world.

Replies to the argument from physics

The Stanford Encyclopedia of Philosophy^[6] and the New Catholic Encyclopedia^[64] give two possible replies to the above objections. The first reply is that the mind may influence the distribution of energy, without altering its quantity. The second possibility is to deny that the human body is causally closed, as the conservation of energy applies only to closed systems. However, physicalists object that no evidence exists for the causal non-closure of the human body.^[65] Robin Collins responds^[66] that energy conservation objections misunderstand the role of energy conservation in physics. Well understood scenarios in general relativity violate energy conservation and quantum mechanics provides precedent for causal interactions, or correlation without energy or momentum exchange.^[67] However, this does not mean the mind spends energy and, despite that, it still not exclude the supernatural.

Another reply is akin to parallelism—Mills holds that behavioral events are causally overdetermined, and can be explained by either physical or mental causes alone.^[68] An overdetermined event is fully accounted for by multiple causes at once.^[69] However, J. J. C. Smart and Paul Churchland have pointed out that if physical phenomena fully determine behavioral events, then by Occam's razor an unphysical mind is unnecessary.^[70]

Robinson suggests that the interaction may involve dark energy, dark matter or some other currently unknown scientific process.^[9] However, such processes would necessarily be physical, and in this case dualism is replaced with physicalism, or the interaction point is left for study at a later time when these physical processes are understood.

Another reply is that the interaction taking place in the human body may not be described by "billiard ball" classical mechanics. If a nondeterministic interpretation of quantum mechanics is correct then microscopic events are indeterminate, where the degree of determinism increases with the scale of the system (see Quantum decoherence). Philosophers Karl Popper and John Eccles and physicist Henry Stapp have theorized that such indeterminacy may apply at the macroscopic scale.^[71] However, Max Tegmark has argued that classical and quantum calculations show that quantum decoherence effects do not play a role in brain activity.^[72] Indeed, macroscopic quantum states have only ever been observed in superconductors near absolute zero.

Yet another reply to the interaction problem is to note that it doesn't seem that there is an interaction problem for all forms of substance dualism. For instance, Thomistic dualism (the dualism of Thomas Aquinas) doesn't obviously face any issue with regards to interaction.^[73]

Thomas Breuer in 1994 had proven^[74] that physical theories valid for the whole universe are impossible. Any theory will be wrong when applied to a system which contains the observer himself due to self-reference.^{[dubious – discuss]} This proves that the observer's own body does not follow the same physical laws as the rest of the universe. But other people from the observer's point of view will obey the usual physical laws, so conducting experiments on them would not indicate any divergence from the physical predictions.

Argument from brain damage

This argument has been formulated by Paul Churchland, among others. The point is that, in instances of some sort of brain damage (e.g. caused by automobile accidents, drug abuse, pathological diseases, etc.), it is always the case that the mental substance and/or properties of the person are significantly changed or compromised. If the mind were a completely separate substance from the brain, how could it be possible that every single time the brain is injured, the mind is also injured? Indeed, it is very frequently the case that one can even predict and explain the kind of mental or psychological deterioration or change that human beings will undergo when specific parts of their brains are damaged. So the question for the dualist to try to confront is how can all of this be explained if the mind is a separate and immaterial substance from, or if its properties are ontologically independent of, the brain.^[75]

Property dualism and William Hasker's "emergent dualism"^[76] seek to avoid this problem. They assert that the mind is a property or substance that emerges from the appropriate arrangement of physical matter, and therefore could be affected by any rearrangement of matter.

Phineas Gage, who suffered destruction of one or both frontal lobes by a projectile iron rod, is often cited as an example illustrating that the brain causes mind. Gage certainly exhibited some mental changes after his accident. This physical event, the destruction of part of his brain, therefore caused some kind of change in his mind, suggesting a correlation between brain states and mental states. Similar examples abound; neuroscientist David Eagleman describes the case of another individual who exhibited escalating pedophilic tendencies at two different times, and in each case was found to have tumors growing in a particular part of his brain.^[77][78]

Case studies aside, modern experiments have demonstrated that the relation between brain and mind is much more than simple correlation. By damaging, or manipulating, specific areas of the brain repeatedly under controlled conditions (e.g. in monkeys) and reliably obtaining the same results in measures of mental state and abilities, neuroscientists have shown that the relation between damage to the brain and mental deterioration is likely causal. This conclusion is further supported by data from the effects of neuro-active chemicals (such as those affecting neurotransmitters) on mental functions,^[79] but also from research on neurostimulation (direct electrical stimulation of the brain, including transcranial magnetic stimulation).^[80]

Argument from biological development

Another common argument against dualism consists in the idea that since human beings (both phylogenetically and ontogenetically) begin their existence as entirely physical or material entities and since nothing outside of the domain of the physical is added later on in the course of development, then we must necessarily end up being fully developed material beings. There is nothing non-material or mentalistic involved in conception, the formation of the blastula, the gastrula, and so on.^[81] The postulation of a non-physical mind would seem superfluous.^{[citation needed]}

Argument from neuroscience

In some contexts, the decisions that a person makes can be detected up to 10 seconds in advance by means of scanning their brain activity.^[82] Furthermore, subjective experiences and covert attitudes can be detected,^[83] as can mental imagery.^[84] This is strong empirical evidence that cognitive processes have a physical basis in the brain.^[85][86]

Argument from simplicity

The argument from simplicity is probably the simplest and also the most common form of argument against dualism of the mental. The dualist is always faced with the question of why anyone should find it necessary to believe in the existence of two, ontologically distinct, entities (mind and brain), when it seems possible and would make for a simpler thesis to test against scientific evidence, to explain the same events and properties in terms of one. It is a heuristic principle in science and philosophy not to assume the existence of more entities than is necessary for clear explanation and prediction.

This argument was criticized by Peter Glassen in a debate with J. J. C. Smart in the pages of Philosophy in the late 1970s and early 1980s.^[87][88][89] Glassen argued that, because it is not a physical entity, Occam's Razor cannot consistently be appealed to by a physicalist or materialist as a justification of mental states or events, such as the belief that dualism is false. The idea is that Occam's razor may not be as "unrestricted" as it is normally described (applying to all qualitative postulates, even abstract ones) but instead concrete (only applies to physical objects). If one applies Occam's Razor unrestrictedly, then it recommends monism until pluralism either receives more support or is disproved. If one applies Occam's Razor only concretely, then it may not be used on abstract concepts (this route, however, has serious consequences for selecting between hypotheses about the abstract).